Changes to event bindings

core#885 makes changes to the event bindings in a breaking but very minimal way. It changes the bindings for only events with no parameters

Consider the following event:

sol! {
    event Incremented();
}

Before

The generated struct was an empty struct like below:

pub struct Incremented { };

After

A unit struct is generated like below:

pub struct Incremented;

Bindings for events with parameters remain unchanged.